<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="utf-8" />
  <title>Z Fighting</title>
</head>

<body onload="main()">
  <canvas id="webgl" width="400" height="400">
    Please use a browser that supports "canvas"
  </canvas>

  <script src="../lib/webgl-utils.js"></script>
  <script src="../lib/webgl-debug.js"></script>
  <script src="../lib/cuon-utils.js"></script>
  <script src="../lib/cuon-matrix.js"></script>
  <script>
    // Zfighting.js (c) 2012 matsuda
    // Vertex shader program
    var VSHADER_SOURCE =
      'attribute vec4 a_Position;\n' +
      'attribute vec4 a_Color;\n' +
      'uniform mat4 u_ViewProjMatrix;\n' +
      'varying vec4 v_Color;\n' +
      'void main() {\n' +
      '  gl_Position = u_ViewProjMatrix * a_Position;\n' +
      '  v_Color = a_Color;\n' +
      '}\n';

    // Fragment shader program
    var FSHADER_SOURCE =
      '#ifdef GL_ES\n' +
      'precision mediump float;\n' +
      '#endif\n' +
      'varying vec4 v_Color;\n' +
      'void main() {\n' +
      '  gl_FragColor = v_Color;\n' +
      '}\n';

    function main() {
      // Retrieve <canvas> element
      var canvas = document.getElementById('webgl');

      // Get the rendering context for WebGL
      var gl = getWebGLContext(canvas);
      if (!gl) {
        console.log('Failed to get the rendering context for WebGL');
        return;
      }

      // Initialize shaders
      if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
        console.log('Failed to intialize shaders.');
        return;
      }

      // Set the vertex coordinates and color (the blue triangle is in the front)
      var n = initVertexBuffers(gl);
      if (n < 0) {
        console.log('Failed to set the vertex information');
        return;
      }

      //Set clear color and enable the hidden surface removal function
      gl.clearColor(0, 0, 0, 1);
      gl.enable(gl.DEPTH_TEST);

      // Get the storage locations of u_ViewProjMatrix
      var u_ViewProjMatrix = gl.getUniformLocation(gl.program, 'u_ViewProjMatrix');
      if (!u_ViewProjMatrix) {
        console.log('Failed to get the storage locations of u_ViewProjMatrix');
        return;
      }

      var viewProjMatrix = new Matrix4();
      // Set the eye point, look-at point, and up vector.
      viewProjMatrix.setPerspective(30, canvas.width / canvas.height, 1, 100);
      viewProjMatrix.lookAt(3.06, 2.5, 10.0, 0, 0, -2, 0, 1, 0);

      // Pass the view projection matrix to u_ViewProjMatrix
      gl.uniformMatrix4fv(u_ViewProjMatrix, false, viewProjMatrix.elements);

      // Clear color and depth buffer
      gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

      // Enable the polygon offset function
      gl.enable(gl.POLYGON_OFFSET_FILL);
      // Draw the triangles
      gl.drawArrays(gl.TRIANGLES, 0, n / 2);   // The green triangle
      gl.polygonOffset(1.0, 1.0);          // Set the polygon offset
      gl.drawArrays(gl.TRIANGLES, n / 2, n / 2); // The yellow triangle
    }

    function initVertexBuffers(gl) {
      var verticesColors = new Float32Array([
        // Vertex coordinates and color
        0.0, 2.5, -5.0, 0.4, 1.0, 0.4, // The green triangle
        -2.5, -2.5, -5.0, 0.4, 1.0, 0.4,
        2.5, -2.5, -5.0, 1.0, 0.4, 0.4,

        0.0, 3.0, -5.0, 1.0, 0.4, 0.4, // The yellow triagle
        -3.0, -3.0, -5.0, 1.0, 1.0, 0.4,
        3.0, -3.0, -5.0, 1.0, 1.0, 0.4,
      ]);
      var n = 6;

      // Create a buffer object
      var vertexColorbuffer = gl.createBuffer();
      if (!vertexColorbuffer) {
        console.log('Failed to create the buffer object');
        return -1;
      }

      // Write the vertex coordinates and color to the buffer object
      gl.bindBuffer(gl.ARRAY_BUFFER, vertexColorbuffer);
      gl.bufferData(gl.ARRAY_BUFFER, verticesColors, gl.STATIC_DRAW);

      var FSIZE = verticesColors.BYTES_PER_ELEMENT;
      // Assign the buffer object to a_Position and enable the assignment
      var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
      if (a_Position < 0) {
        console.log('Failed to get the storage location of a_Position');
        return -1;
      }
      gl.vertexAttribPointer(a_Position, 3, gl.FLOAT, false, FSIZE * 6, 0);
      gl.enableVertexAttribArray(a_Position);
      // Assign the buffer object to a_Color and enable the assignment
      var a_Color = gl.getAttribLocation(gl.program, 'a_Color');
      if (a_Color < 0) {
        console.log('Failed to get the storage location of a_Color');
        return -1;
      }
      gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, FSIZE * 6, FSIZE * 3);
      gl.enableVertexAttribArray(a_Color);

      return n;
    }
  </script>
</body>

</html>